Problem: the Media Browser lists .mxf files, but File > Import's dialog
filtered them out and media::is_importable refused them, although the
pinned FilmCraft codecs behind crates/media already demux MXF (AVC,
MPEG-2 / XDCAM / D-10, VC-3 / DNxHD, ProRes, PCM audio) by content.
Fix: add "mxf" to VIDEO_EXTENSIONS and to the File > Import and Set
Proxy dialog filters. Tests check MPEG-2 4:2:2 long-GOP + 24-bit PCM and
DNxHD 36 MXF against ffmpeg frames (>35 dB, measured 64-67 dB), audio
level, and that the Import dialog offers MXF. AVI stays open: FilmCraft
has no AVI demuxer.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>